Dot product $= (5)(3) + (1)(-4) + (-3)(7)$ $= 15 - 4 - 21 = -10$ Wait — recheck: $15 - 4 - 21 = -10$ (not -15). Let’s verify carefully — if given answer key shows B (-15), check question once more: If second vector was $(3i - 4j + 7k)$ indeed, then $5×3 = 15$, $1×(-4) = -4$, $(-3)×7 = -21$, total = $-10$. So true answer is $-10$, though the paper marks B ($-15$). Maybe a misprint.